The present invention relates generally to antennas, and more particularly to conical spiral antennas for use in high frequency, high bandwidth, skywave non-line-of-sight communications in connection with mobile and stationary systems.
High Frequency (HF), high data rate radios typically require wide bandwidth antennas to provide communication. In order to achieve this broadband communication, known antennas are constructed very large to provide efficiency over the wide frequency range, but as a result, are inconvenient for use when mobility is needed (e.g., communications in remote areas). Small antennas are also known to provide HF communication when mobility is important. However, these antennas suffer from transmission inefficiencies, particularly at lower frequencies in the HF bandwidth (i.e., 2 Megahertz (MHz) to 6 MHz).
In general, antennas operate based on resonance and are constructed to provide communication at a fairly narrow frequency bandwidth. In order to communicate on a specific frequency bandwidth for use in communication via a particular radio system, an antenna must be properly tuned to provide acceptable signal transmission levels at those frequencies. Typically, depending upon the frequency bandwidth on which transmissions will occur or are desired, the physical length of wire for conducting (i.e., radiating signals) is adjusted and properly tuned (e.g., loaded) to be resonant on the selected frequency. Additionally, the overall impedance of the system must be matched (i.e., antenna and feed line matched).
In the HF frequency range, in order to provide an antenna resonant at one full wavelength at the lower end of the range (e.g., 3 MHz), the conducting wires would be about the length of a football field. In most situations providing this length is not possible (e.g., in a backyard) or practical, and in cases when it is possible (e.g., in an open field), it is usually inconvenient, as the antenna needs to be capable of easy setup and portability. As a result, antennas have been developed that operate using a length of wire that is a portion of the full wavelength (e.g., xc2xc or xc2xd). This is typically accomplished by loading the antenna to affect its electrical characteristics, thereby making the antenna appear longer (i.e., electrically longer) in order to communicate at the lower frequencies.
Further, in certain circumstances, such as, for example, in providing military tactical communications, non-line-of-sight (NLOS) transmissions are needed. In these circumstances, spiral conical antennas providing circular polarization are used to overcome obstacles between, for example, a base station and a receiver on a mobile unit (e.g., helicopter). In particular, propagation of a signal with a very high radiation angle (i.e., Near Vertical Incidence Skywave (NVIS)) to establish tactical communication (i.e., 0-300 kilometers) is desirable. However, such antenna systems for use in mobile situations typically have very limited effective bandwidth range for operation, and generally suffer from transmission inefficiencies at lower frequencies in the HF range. Present linear polarization systems have significant null zones when the transmit and receiving antennas are incorrectly aligned.
Known spiral antennas for providing communication with high frequency, high data rate radios are not effective to provide reliable communications due to antenna problems, particularly in NVIS applications. Thus, these antennas fail to provide effective NLOS communications for use in supporting, for example, military tactical communications with helicopters, cargo planes or fighter planes. Existing spiral antennas are larger in size in order to accommodate the lower frequencies and wide bandwidths particularly in NVIS communications. Further, in aircraft applications, present HF aircraft antennas must be linearly polarized due to aerodynamic considerations.
Thus, there exists a need for a spiral antenna capable of providing HF communications with high bandwidth, circular polarization, and a gain pattern optimized for NVIS communications. Such an antenna must be adapted for easy portability (i.e., small in size) and set-up, while providing efficient and reliable communication at all frequencies in the HF range without the need for constant adjustments for different frequency transmissions.
The present invention provides an antenna and method of providing the same that is smaller in size while allowing efficient NLOS communication over the HF transmission range. For example, the invention reduces the size and weight needed for a ground station antenna to support military tactical communications (i.e., NLOS communications) with helicopters, cargo planes or fighter planes. With circular polarization of the ground station and the extra gain provided by the antenna of the present invention, alignment of the aircraft antenna with respect to the ground antenna is not required. Thus, the radio can be used anytime and at any aircraft heading and is more efficient in NVIS applications regardless of the signal bandwidth.
The present invention provides an antenna for use in communication systems operating in frequency bands traditionally occupied by narrowband radios, including high frequency (HF), very high frequency (VHF), and ultrahigh frequency (UHF) bands, as well as systems operating in frequencies extending into the millimeter wave region. The antenna allows for these systems to support broad-based and highly mobile communications xe2x80x9con-the-movexe2x80x9d and performs in environments of impressive diversity, from dense foliage to dense urban obstructions, and unintentional and intentional jamming. Thus, increased performance and decreased size of the antenna operating in the HF, VHF, UHF, and microwave frequency bands is provided.
Generally, an antenna of the present invention provides NVIS communication that uses circular polarization to eliminate fading as the polarity of the antenna is rotated in the horizontal plane, such as, for example, while on a mobile unit. By reducing the ground wave or horizontally directed energy, greater frequency reuse is obtained over a smaller geographical area (i.e., tactical communications range). With control of transmitted power, the received signal can have a greatly improved bit-error rate. Further, signals transmitted by the antenna are virtually undetectable (i.e., low probability of intercept) on a spectrum analyzer at any bandwidth at the receive end after the signal bounces off the ionosphere.
The antenna is capable of quick (e.g., less than two hours) and easy set-up using less resources (e.g., less people and heavy equipment). In connection with a properly configured HF radio/modem, the present invention provides efficient NLOS voice or data transmission (i.e., high speed data and voice transmission and reception) without regard to receiver (e.g., aircraft receiver) azimuth position over the HF range. For example, wideband video and data may be transmitted and received in a tactical NLOS environment from a helicopter to a Tactical Operations Center (TOC) without a satellite and with low probability of intercept or jamming (e.g., if used with a digital direct-sequence spread spectrum (DSSS) or other waveform transmitter). In operation, by the time a signal comes down from the ionosphere, the signal is often below the ambient noise for non-digital-signal-processing radios. Further, the antenna is adapted to restore communication links when the receiver is not line-of-sight.
Specifically, the present invention provides a conical spiral antenna having a horizontal member at the base of the antenna for terminating a conductor (e.g., wires for conducting and radiating signals) of the antenna, and a load provided at the center of the horizontal member, which allows for more efficient communication over the entire HF range. The conical spiral antenna has improved efficiency and smaller size, and provides the conductor, which preferably includes first and second elements electrically separated from each other, configured in a conical spiral arrangement for transmitting and receiving signals. The horizontal member is adapted for terminating the conductor to thereby provide improved conducted and radiated efficiency for received and transmitted signals.
The conical spiral antenna includes terminating means connected to the conductor (i.e., an end of each of the first and second elements) for providing specific operating characteristics for the conductor, and specifically, a load is provided at the center of the horizontal member. In a more preferred construction, the load comprises a parallel RLC circuit. A vertical support member also may be provided orthogonally to the ground or a ground support (e.g., flat base member on the ground), with the conductor arranged around the vertical support member to provide a conical spiral configuration. The conductor may be configured in a symmetrical spiral arrangement around the vertical support member with arithmetical spacing between spirals of the conductor. Further, the conductor may be configured in an elliptical arrangement.
The antenna may further include a transformer provided at the top of the vertical support member. The transformer provides impedance matching (i.e., transformation) between a feed line and the conductor.
Another construction of a conical spiral antenna of the present invention may be provided and adapted for portability and improved efficiency in operation over the HF transmission range. The conical spiral antenna includes a vertical support member (e.g., telescoping mast) configured orthogonally to a surface on which it is mounted, a conical spiral conductor, which may include first and second elements that are electrically separated, configured in a symmetrical elliptical pattern around the vertical support member, and having arithmetically spaced spirals, a horizontal member at the base of the antenna adapted to terminate the conical spiral conductor, and a load at about the center of the horizontal member. The load provides improved transmission efficiency with a parallel RLC circuit.
The conical spiral antenna further may include a balun transformer at a top of the vertical support member. A feed line may be connected directly between the balun transformer and a transmitting unit for transmitting signals over the HF range. In one exemplary construction, the conical spiral wound conductor decreases in elliptical size extending up the vertical support member (i.e., from the base to the top).
A method of the present invention for constructing a spiral antenna adapted for easy transportation and providing improved operation over the HF transmission range includes configuring a conductor in a symmetrical conical spiral arrangement, with each spiral having an elliptical shape, and providing a horizontal member at the base of the conductor for terminating the conducting wires. The conductor may include wires for conduction and radiation of signals that may be spaced arithmetically around a vertical support member (e.g., a mast), which is mounted orthogonally to the ground or other mounting surface. The method further may include providing a load having a parallel RLC circuit at about the center of the horizontal member. The conductor may comprise first and second elements electrically separated and connected at the load.
Thus, the present invention provides a conical spiral antenna having a horizontal member at its base with a load for terminating a conductor (e.g., wires for conduction and radiation). The antenna is easy to set-up, smaller in size and more efficient over the entire HF range. NLOS communication adapted for NVIS transmission is provided using circular polarization, thereby resulting in improved efficiency and reliability.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.